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Thread: Peavey Classic Tremolo Circuit Not Working

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    Stray Cap DrGonz78's Avatar
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    Peavey Classic Tremolo Circuit Not Working

    I am trying to get the tremolo working on a Peavey Classic amp. I had original thought it might be a bad sps-953 because when I first checked the circuit all the voltages were jumping around. I did replace the faulty sps-953 (replaced with 2n6520) and all the voltages in the circuit are now stable. Still my tremolo circuit is not working and has the same symptom. Also, to note that I have ruled out that the sps-690 as being faulty and have put a 2n4249 to replace. I have also changed SPF-576 to a 2n5461, but realizing it was not the problem put the old one back. All the parts were looked up as reliable replacements using the Peavey transistor cross reference guide. Also, have to say that every electrolytic cap was replaced as well. Tube drivers replaced, new multican filter cap and the amp sounds great, just no tremolo.

    So the symptom occurs when you turn up the depth pot for the tremolo and the volume of the amp goes to zero. Just as if it is shunting the signal to ground much like a backwards master volume pot. Now I did some signal tracing and signal reaches the pot and (obviously) connects through the pot. As I turn up the pot, signal can be traced on the input to the pot and is not effected when I turn it up. However, after the pot the signal goes away as the pot is increased. So, I tested the pot and even took it apart to have a close look at the wipers. It all looks fine and has me stumped. I am not using a footswitch but I believe the tremolo should work without the foot switch. I have been reading up on the way that JFETs work and I understand that it is used for switching purposes.

    The depth pot passes signal to the JFET SPF-576 on the Source leg of the transistor. I measured the idle voltage of the 576 S=5mvDC D=.7mvDc G=.9mvDC. Another thought about the SPF-576 is that it is not marked as that in the peavey transistor guide. Actually the only one in there that made sense was SPS-576, so I took that as the type I have here in this amp. I guess it could be a bad part and the 2n5461 is really not the right cross reference for this JFET. New one did not change the behavior so I put the old one back. I did test both with meters and the old one appears to be fine.

    I am posting the schematic and a picture of the schematic with my notes. I am thinking that something is not right in the circuit(obviously), but just can't seem to track it down. On the schematic there is reading that says .6vDC connecting to the base of sps-953 and I get 7.5vDC on my circuit. Looking at the schematic it might really say 6volts and the .6 could be a dot on the copy. But if that base is supposed to be .6vdc then I think that is my problem. The caps could be leaky or something else. Check my picture for added comments. I hope someone can help cuz I am scratching my head right now.

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    Supporting Member Jazz P Bass's Avatar
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    Can you post the complete schematic?

    The high voltage on Q953 is a major issue.

    Have you tried grounding the point marked FS?

    Double check the 470K to ground.

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    Stray Cap DrGonz78's Avatar
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    Sorry that schematic must be up online here but part of the crashed files. Try this one out... I will do some further test and try grounding out FS 5, which is part of the foot switch. Also I think I need to pull up a leg on those resistors to test them out circuit.
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    Your voltages look like they would make sense if Q953 were installed backwards or the wrong transistor... Just noticing that the collector is a diode drop from the base...

    Checked that theory with a circuit simulator, and my hypothesis did not hold water. I'm very curious why that strange reading there.

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    Last edited by dwmorrin; 11-30-2013 at 12:10 AM.

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    Stray Cap DrGonz78's Avatar
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    Yeah I am at a loss too lol. I grounded out FS5 and checked to see the voltage at the base of 953 and it was at 7.5vdc still. I just popped up a leg on each the the 2.2meg and 470k resistor. Both measure well within tolerance, 2.2m and 467k were the readings. I will double double check that the 2n6520 is the exact one and possibly it's one of those situations that I have the 2n6520ag model where they changed something. Not really thinking that is the case either but at this point any idea to try is valid. I am also gonna test that .1 cap (the one running in parallel) to see what it is reading or if it is bad possibly.

    Edit: Hold the phone!! lol. I think I made an oversight here looking at the schematic I just realized I need a 2n6520 npn type and I put in a pnp type. So that explains a whole lot. I was really careful but not careful enough. This makes sense then why it is not working, sorry.

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    Last edited by DrGonz78; 11-30-2013 at 01:05 AM.
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    You said you replaced the 953, which is NPN on the schematic, with a 2N6520... which appears to be a PNP, no?
    Maybe the voltages were just jumping around because the oscillator was on, and now it is off.

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    Stray Cap DrGonz78's Avatar
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    Yeah I had overlooked the pnp/npn part of the schematic and the 2n6520 has the part option to be pnp or npn type.
    http://www.mouser.com/ds/2/308/2N6515-D-102947.pdf

    I am not sure maybe the voltages jumping around were an indication that the circuit was working, but the original 953 definitely was shorting out on my meter. I have a mps8097 just right here by chance and it's a good replacement type, npn onlyfor this one. Now if the voltages are jumping around again then yes that would make me think the circuit is working and it was not even any of the transistors. Still I stand by the fact that the 953 had a intermittent leg on my meter. I will report back my findings. At least now I will see what readings I get with the right part and if the depth pot still shunts the signal I will be tracing the signal upstream to find my culprit. Thanks again ya'll.

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    Stray Cap DrGonz78's Avatar
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    Okay back on track as far as the DC voltages are concerned. Too many words to express the changes, but 7.5vdc went down to .6vdc on the base of the 953. Refer to my new updated pic to look at the changes in voltages in the tremolo circuit. The voltages are not fluctuating at all, and at the very start even the main supply 23vDC was fluctuating. All of the voltages were fluctuating before and now they are good. Still tracing the signal after turning up the depth pot and it will not pass signal. I guess I need to ensure that the rate pot is not bad. Still thinking that the 576 JFET might be a culprit too. Anyone got some ideas or tests I could try? Thanks again for helping and I appreciate how this site gets me connecting my brain cells back together.

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    Edit: BTW where do I trace signal in the tremolo circuit? I mean should there be signal on the emitter leg of the 953 transistor? I also only trace signal on the source leg of the 576 when the depth pot is turned down. Shouldn't I be tracing signal on the drain or source of the 576? Still unsure how to proceed, but now my voltages are better at least. The right parts help, but my original problem seems to be unrelated to the faulty 953.

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    Last edited by DrGonz78; 11-30-2013 at 02:29 AM.
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    Have you tried adjusting the 50k bias pot? What does the LFO signal look like there? Is the 576 just stuck on? Maybe the LFO is biased where it's keeping the 576 on all the time, instead of the desired on/off action.

    You should be able to verify the rate pot by scoping the LFO signal. I would think you should see it clearly at the 14V node, at the top of the bias pot.

    I could be wrong, but I think the action of the tremolo is that the FET turns the signal down as it conducts, and full signal when it is "open."
    So removing the FET should give you full signal regardless of the depth control.
    Figuring out what's going on at the FET's gate terminal should help determine if the LFO is to blame.
    Test the FET while you've got it pulled.

    Just saw your latest edits. I wouldn't worry so much about tracing the audio signal as much as checking out the LFO. Set your scope to view very low Hz. If you don't see the LFO, well that's your problem then. We've got to figure out why you're not oscillating.

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    Last edited by dwmorrin; 11-30-2013 at 03:27 AM.

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    Quote Originally Posted by DrGonz78 View Post

    Edit: BTW where do I trace signal in the tremolo circuit? I mean should there be signal on the emitter leg of the 953 transistor?
    If I'm reading the schematic correctly, the 953 emitter is tied to ground. You should not be getting signal there. One leg of the FET should also be at ground. Check those grounds.

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    Okay yeah I was wondering what the circuit is actually doing to the signal, this is starting to clear it up a bit. I need to take a break at the moment to eat and just get away from it for a few. I will try removing the fet later and scoping for LFO as well. I just did one last thing just now and it is something I have never encountered before. I wanted to trace a signal to the leg of that 14v junction after the bias pot. Just to mention, I did adjust the bias pot and the voltage there is now 14.4volts. I am running amp with a 4ohm load and no speaker. I have been tracing signal at the moment with just my DMM, and will get out the scope when I get back. But while I pumping some Creedance from the radio into the amp and turned up the volume, I heard the music. I am not sure how that occurs and it had me kinda giggling. Seems like the tubes or output area was where the music was eminating. So where or how is sound coming out of the amp? lol

    Anyhow I will return soon and do some more testing. Will post some results and hopefully get this darn thing working, thanks.

    Edit: Just to mention when I was tracing the AC signal just now at that 14v junction, I was getting very sporadic readings as if it was oscillating. I mean music does jump around rather than using a signal generator, but I can tell the difference it was big. Hopefully I can get my computer pumping out my signal tone generator later as it was giving me grief the other day.

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    Stray Cap DrGonz78's Avatar
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    Well I struggled looking for LFO on the circuit and that might be that it is simply not there. Or it could be that I am not sure how to get my scope to read LFO. I guess this is the first time looking for LFO on a circuit and just not sure how to do it right now.

    On the bright side I did pull the rate pot and cleaned up the legs and solder connections. This did the trick as now when I turn up the depth pot the incoming signal is still there strong. So I guess these are two unrelated faults that we have been working to solve. I even tried grounding the FS5 pin to make sure that was not the problem and it still would not trigger the tremolo. I need to also clarify that the sps690 was bad as I tried putting it back in to check voltages. That triggers the huge shifting voltage problem that was there before and both the 953 & 690 transistors test very high on the HFE. One thing to note is that testing the 2n4249 in the hfe tester was backwards to the 690 and the 953. Which makes me wonder it the 2n4249 has the pin out backwards or something. I know that on the peavey cross reference sheet that it does not state a difference in base layout. Not much info online about 2n4249 except one old datasheet that does not even tell you the pin layout. My thoughts on this is that I should have went with a 2n5086 instead of the 2n4249, oh well.

    http://datasheet.seekic.com/PdfFile/...248_2N4249.pdf

    Well tomorrows always another day... I guess I just need understand how read LFO on a scope and where to look for it on the schematic. Thanks for all the help so far this amp has proven to be a project with many lessons to learn.

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    Lifetime Member Enzo's Avatar
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    LFO just means low frequency oscillator - it makes a really slow waveform. You want to see it on a scope? You clip to someplace the LFO signal might be, like the top of the bias pot, or the collector of 690 where it says 8v. Your scope trace ought to move up and down at the LFO speed. Don;t bother trying to resolve it into an actual sine wave on the screen, it is sufficient to see it moving a few volts. Nothing cosmic about it.

    Grounding FS5 just kills the oscillation, leave FS5 floating and the LFO should be free running.

    Note the bottom end of the bias pot is ground.

    How does it work? The 690 and its pal form an oscillator, the signal from which is fed to the bias pot. SKip over to the JFET 576. A JFET is a low resistance unless turned off at its gate with a voltage. SO the LFO is turning the JFET off and on. The JFET is in parallel with the depth pot to the extent the depth is turned up. SO when you report that turning the depth up causes the signal in the amp to go away, it tells me the JFET is stuck on. Either it is bad, or its gate is being held on. But the depth pot itself was working.

    Look at the bias control. It looks like a "volume" control for the LFO output, and sorta is, but it also has 14vDC across it, which means the setting of the control affects the DC voltage at the gate of the JFET. The bias is to be set so the range of voltage the LFO produces are compatible with the gate voltages the JFET needs to turn off and on. If the bias pot is set too high, the voltage is on the gate of the JFET all the time, so it stays on. And that means whenever you turn up the depth, it will squash the signal. Set it too low and you get no trem or maybe just the peaks of the trem. The way to do it is get the LFO running, then adjust the bias up and down listening to the effect. Center the control in the band where it makes sense.

    If you changed the JFET, MAKE SURE the leads are in the right holes. There is no standard pinout for those things. Take your new part, measure resistance between the three legs while not in circuit. Two legs will have a low resistance between them. I expect a 100 ohm more or less as a rule. Those two are the source and drain, which are interchangable here. The other one is the gate. Now trace the circuit ON THE BOARD. You have three holes. One is grounded, and one is wired to the depth pot wiper. Those are source and rain holes. The gate lead hole connects to the pair of 1M resistors, one of which continues on to the bias control. Once you know which hole is which, and which JFET leg is which, THEN insert the JFET so the appropriate legs are in the appropriate holes.

    The bias control will have a substantial effect on the trem effect, but it shouldn;t really influence the LFO itself.


    Now your LFO. Look at the 953, the emitter is grounded, so the base ought to sit at about 0.6v - the dot belongs there. You had 7v. SO maybe the 470k is open or the 953 is shorted. But you have since cured that. Look at the bias pot. It is 50k, and is joined by the 1k and 10k above it, all in series and across 24v. You got roughly the 14v and 23v on either end of the 10k?

    You speak of voltage hopping all over, well most likely that is your LFO. The music signal doesn't come anywhere near the LFO. The LFO is a simple oscillator, and that is all you will find there, a slow speed signal going up and down.

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    Stray Cap DrGonz78's Avatar
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    Houston? Tremolo Boosters Set? We have lift off!! It is fixed and biased perfectly now. I think my troubles had to do with my lack of knowledge dealing with the tremolo circuit(obviously). Of course you guys really set me straight. Thanks to Enzo, dwmorrin and Jazz for helping me through this one! Enzo you always have this way of explaining the circuit in a way that a newbie amp tech can comprehend. dwmorrin you really helped me figure out about the circuit as well and for that I am very grateful.

    I think the 2n4249 as a replacement for the 690 has an opposite pin out or my part is bad. I am betting the 2n4249 that I have is backwards and might have worked had I installed it that way. So it turns out the circuit was probably fine the whole time and I really just did not understand voltage readings. Plus I was clueless about what the bias was doing to activate the FET. Now I feel silly but enlightened at the same instance. This site is the reason I am now troubleshooting better on my own, but in cases like these it is proof that I have a lot to learn. Thanks for all the help guys!! Don't eat too many left over turkey sandwiches!!

    BTW the 2n5461 has the same pin out as the SPF-576(aka SPS-576) and Enzo thanks for the tip on measuring the legs to find the Source and Drain. To note the resistance between source and drain on the 2n5461 is really high like 300k or above, but the SPF-576 had resistance around 300 ohms. I must have had the circuit working either way but my bias adjustment is what made the FET on all the time. I forgot that I increased the bias pot resistance thus turning on the FET, and mistakingly thought that cleaning the pot leg connections was the reason. Oh well I learned something after all and that is what really matters.

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    I've been working on a classic a&b lately. Had to do a lot of repairs to the power section to get it to work. Got it making good sound but the tremolo wouldn't work at first. After awhile of playing it the tremolo kicked in and it was fantastic!! Then after awhile it faded out and quit again. After read this thread I have a better idea how this works. Can a fet be "stuck". Or do they just go bad. Adjusting the 50k bias pot didn't seem to have an affect on the inoperable tremolo.

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    Lifetime Member Enzo's Avatar
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    Hi Jimmy. I am glad this old thread was helpful, but please start a new thread for your amp. It gets confusing when more than one repair is in the same thread.

    If function comes and goes it suggests a poor connection somewhere in the circuit.

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